Mast cells play a pivotal role in the pathogenesis of asthma and other allergic diseases. These reactions are generally initiated by antigen-dependent aggregation of the high affinity IgE receptor (Fc-epsilon-RI) expressed on the cell surface and subsequent release of pro-inflammatory mediators (e.g. histamine, prostanoids, proteases and cytokines). We have demonstrated that interferon-gamma up-regulates the high affinity receptor for IgG (Fc-gamma-RI) on cultured human mast cells and that aggregation of these receptors leads to mast cell mediator release. An objectof this study is to delineate the signaling pathways of Fc-epsilon-RI and Fc-gamma-RI receptors leading to the release of inflammatory mediators from human mast cells. The current model for the signaling pathway linking Fc-epsilon-RI to mast cell activation has largely been based on studies conducted in RBL 2H3 cells. RBL 2H3 cells are a transformed rodent cell line, which may not be, in all aspects, reflective of the mature human mast cells. The signaling pathways linking the Fc-gamma-RI receptor to mast cell activation have yet to be delineated. It is now possible to grow human mast cells from a CD34-positive, pluripotent cell population in primary culture with sufficient number and purity to study defined signaling events. Thus, we are now examining specific signaling pathways involved in Fc-epsilon-RI- and Fc-gamma-RI- dependent inflammatory mediator release from human mast cells.